Synthesis of an RNA having a modified 2′-hydroxyl group requires a ribonucleoside monomer intermediate having a modified 2′-hydroxyl group. Main conventional methods of synthesizing this monomer intermediate are two: One is a method wherein the 2′- or 3′-hydroxyl group of a ribonucleoside of which the base site and the 5′-hydroxyl group are protected is converted to alkoxide with NaH, and then the ribonucleoside is subjected to a reaction with alkyl halide (Non-Patent Document 1). In this method, since in addition to a 2′-modified product a 3′-modified product, which is a positional isomer, is produced in a large amount, many cases have difficulties in separating them. In the other method, a ribonucleoside derivative of which the base site and the 3′- and 5′-hydroxyl groups are protected is reacted with NaH to alkylate the hydroxyl group at the 2′-position to form alkoxide, or is reacted with alkyl halide in the presence of a strong organic base such as BEMP (Non-Patent Document 2). In this method, a protecting group that can simultaneously protect both 3′- and 5′-hydroxyl groups is necessary, and, in general, a 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl (TIPS) group or a di(tert-butyl)silanyl (DBS) group is used. However, these protecting groups are unstable to reagents such as NaH and BEMP and thereby have a major problem that by-products are produced. Since the basic conditions employed in both synthesis methods described above are strict, it is generally impossible to incorporate a functional group such as methyl ester or ethyl ester into an alkyl halide derivative that is used as a modifying group-inducing reagent. Consequently, in order to maintain the stability, only a group having large steric hindrance, such as tert-butyl ester, can be incorporated as an ester skeleton.    [Non-Patent Document 1] Richard H. Griffey, Brett P. Monia, Lendall L. Cummins, Susan Freier, Michael J. Greig, Charles J. Guinosso, Elena Lesnik, Sherilynn M. Manalili, Venkatraman Mohan, Steven Owens, Bruce R. Ross, Henri Sasmor, Ed Wancewicz, Kurt Weiler, Patrick D. Wheeler, and P. Dan Cook, J. Med. Chem. 1996, 39, 5100-5109.    [Non-Patent Document 2] Grotli, M., Douglas, M., Beijer, B., Garcia, R. G., Eritja, R., and Sproat, B., J. Chem. Soc., Perkin Trans. 1, 1997, 2779-2788.